The present invention relates to communications systems, and more particularly, to time division, multiple access digital bus communications systems.
The need for the efficient interconnection of computers and computer-related devices has become increasingly apparent with the development of computer technology. In particular, multi-processor systems typically include a number of computers, often widely distributed, which are interconnected by a data bus to remote subscriber terminals and associated peripheral devices. With the developing technology, such terminals and devices have become diverse and capable of wide distribution. The interconnection between such devices has become critical to the system use and performance, particularly in view of the need for adequate digital communication bandwidth, fast response time, high reliability, and the requirement for full access to all coupled but physically distributed devices. Conventional systems typically utilized circuit and message switching techniques. In addition, packet switching techniques have been utilized, although such systems have required relatively expensive store and forward nodes, which raise cost and reliability problems.
Primarily, the conventional time division multiple access (TDMA) bus communications systems have relied on time slotting and assignments of these slots to transmitting subscribers, either permanently or at sign-on to the TDMA system. For example, see U.S. Pat. No. 3,851,104, Willard, et al. While such systems are efficient for a population of high duty cycle traffic, large numbers of low duty cycle users, such as intercommunicating computers, quickly overburden many of the prior art TDMA systems having slots uniquely assigned to various subscribers.
In view of this problem, bus contention protocols have been developed to more efficiently utilize the available bandwidth for computer and terminal communications. Generally, contention systems permit a user to gain access to the entire channel bandwidth for his message burst. In operation, a user having a message to send is permitted to transmit his message whenever he is ready. In the event another user is presently transmitting over the channel, or does so during the first user's transmission, then a message collision occurs. The system users must sense these collisions and, in response, retransmit their respective messages. Some systems provide random delays before retransmissions to avoid "butting", i.e. continual collisions between users.
The contention protocols take advantage of the low duty cycle or "bursty" nature of data being transmitted from terminals and computers. For a large subscriber population of bursty users, the law of large numbers ensures that the channel bandwidth is only required to match the average aggregate data transmission rate of the entire population, rather than matching the sum of the peak rates for bursty subscribers as in the noncontention systems.
Several approaches to the contention protocol have been developed in the prior art. For example, the ALOHA system (see N. Abrahamson "The ALOHA System--Another Alternative For Computer Communications", 1970 Fall Joint Computer Conference, AFIPS Conf. Proc. Vol. 37, AFIPS Press, 1970), describes an unslotted system wherein a plurality of remote stations are connected to a central station by way of a single radio channel. The various remote stations contend for the channel and transmit complete packets of data. In this unslotted configuration, the data packets are not synchronized between stations. When a collision is detected by failure to receive an acknowledgement from the central station, each of the transmitting terminals then retransmits its packet. With this configuration, the ALOHA system is relatively efficient for bursty terminals (e.g. a terminal duty cycle on the order of 1%), compared with a system using separate channels for each station. However, a collision detected during a packet transmission results in the waste of the entire overlapping transmission intervals for the terminals (i.e. as much as two packet lengths), limiting maximum efficiency.
Slotted (i.e. systems where the transmitted packets are synchronized from terminal to terminal) ALOHA systems have been suggested to provide an improvement in efficiency compared with unslotted ALOHA systems. In the latter configuration, the channel is divided into time slots of duration equal to the fixed data packet length. Each user is constrained to begin transmission of a data packet at the start of any time slot. When a collision occurs during a slot, the wasted time is equal to a single packet length, resulting in a substantially higher efficiency than the unslotted system.
Another form of slotted, contention protocol communications system is disclosed in U.S. patent application Ser. No. 881,704, assigned to the assignee of the present application.
An unslotted, listen-while-talk (LWT), protocol, baseband communications system is disclosed in U.S. Pat. No. 4,063,220. This system includes a plurality of remote terminals coupled to a two-way communications medium. Prior to transmission, a terminal monitors the medium to detect activity (i.e. to sense whether the medium is currently busy). If the medium is busy, the terminal must defer at least until the current user is finished and the medium is no longer busy. If any terminal determines that the medium is not presently busy, that terminal may attempt to transmit on the medium.
As a user transmits his message, he also monitors the medium. If the user does not detect any errors in his own transmission for the duration of the end-to-end propagation delay along the medium, then he determines that he has gained sole access to the medium and will successfully complete his packet transmission (unless due to noise interference other than from other users competing for the medium). Upon detection of a collision within the propagation delay interval, each colliding user aborts his transmission, and subsequently retransmits his message at a time when he determines that the medium is no longer busy. With this configuration, the time wasted during a collision is only the end-to-end propagation delay interval for the medium, in contrast to an interval equal to a single packet length as in the slotted ALOHA system, and an interval greater than a packet length as in the unslotted ALOHA system.
Although relatively efficient compared with contention systems of the unslotted and slotted ALOHA types, the prior art LWT systems incorporating baseband transmission over a two-way medium generally require bi-directional repeaters located with minimum separations along the medium. This requirement is particularly important for relatively large systems where a terminal must perform the collision detection packet comparison operation with his own very high signal-to-noise ratio transmission with the substantially lower signal-to-noise ratio transmission which might come from a distant subscriber. In order to perform this operation, repeaters are provided to digitally regenerate the baseband signals on the medium, including reclocking of the signals to obtain full period bits with satisfactory rise and fall times. In addition, the repeaters must be spaced relatively close together on a multi-drop medium in order to offset delay distortion. These requirements for close repeater spacing are costly in terms of equipment and reliability. Furthermore, the prior art baseband LWT systems are highly susceptible to low frequency noise often found in industrial and computer environments.
It is an object of the present invention to provide an unslotted radio frequency, multiple access bus communications system employing a listen-while-talk contention protocol.
It is a further object to provide a radio frequency, multiple access bus communications system with listen-while-talk contention protocol over a system having unidirectional inbound and outbound signal paths connecting subscriber terminals.